76 LIMESTONE AREAS IN QUEENSLAND 



especially to an untrained eye. Almost all famous caves 

 of the world occur in limestone. 



But the existence of caves is not the only result of the 

 subterraneous passage of the waters. The rocks below 

 the surface diminish gradually in volume, and concentric 

 depressions form. Many of such depressions are formed by a 

 slow gradual process, but others are the result of violent, 

 although only localised, processes, being caused by sudden col- 

 lapse of the roof of some cave. The shapes of such sinkholes* 

 explain generally the way in which they originated, and 

 very often the deep, abrupt sinkholes form entrances to 

 the subterraneous passages. On a limestone area con- 

 taining caves and sinkholes the rain water falling upon 

 the surface does not flow away in streams, but disappears 

 underground. Xo regular graded valleys are formed. 

 Running water flowing in creeks or rivers from areas of 

 impervious rocks seld,om succeed in eroding an open valley 

 through a limestone area. It generally disappears in 

 fissures or sinkholes, and continues its way in subterraneous 

 passages, reappearing again on the surface where the 

 limestone area ends, or flows often subterraneously to the 

 sea. 



Many of such subterraneous w^atercourses can be 

 followed on the surface, deep sinkholes marking the 

 direction of the caves, but many of them are completely 

 lost to the human eye. It seems probable that in some 

 limestone areas the cave systems are not yet fully 

 developed, and that the waters after some progress in 

 defined streams lose their individuality, and become a 

 part of a general ground water basin, which fills all cavities 

 and fissures to a certain level throughout some limestone 

 areas. Such conditions seem to prevail in limestone areas, 

 which are morphologically young, and which have not been 

 long (geologically speaking) exposed to the disinte- 

 grating agencies. 



During the further evolution, the Refined underground 

 watercourses become more and more general, and prevail 

 completely in morphologically old areas. 



The caves in their young stage show^ everywhere 

 unmistakable proofs of violent mechanical work performed 



* For sinkholes the Servian term •' doline " is most extensively used in 

 scientific literature 



